An anti join returns records that don't have a matching key in the other table. Left Anti returns unmatched rows from the first table. Right Anti returns unmatched rows from the second table. These joins answer exception questions such as which jobs lack a roster route or which roster entries have no job.
The word anti matters: an inner join finds matches, while an anti join finds their absence. The position of each table matters too. Read the business question and name the list whose missing matches you need before opening the dialog.
Power Query first produces a nested column. A left anti result can drop that empty nested column. A right anti result needs the right fields expanded so the surviving records are visible. Missing keys require a business rule: two nulls may match each other, but an unknown identifier should usually enter a separate data-quality review. Clean the keys before interpreting the exception count.